You Wear It Well: How sea urchins erode reef rock to form pits to nestle in

Sea urchins are often so snugly nestled into cavities in rocky temperate reefs that researchers have long wondered if they excavate these pits themselves. Marine ecologist Michael Russell, a professor known simply as “Dr. Mike” to his students at Villanova University, U.S., went in search of data to evaluate this hypothesis. In what must have been a supreme test of patience, for over a year he monitored the activities of purple sea urchins, Strongylocentrotus purpuratus, on flattened rock surfaces in the lab. I interviewed Russell via email to find out more about his PLOS ONE study.

Russell is here pictured in the same Vancouver Island tidepool – back in 1981, and again in 2007! He has been studying tidepool ecosystems for over 35 years.

What led you to study marine ecology?

MR: I got hooked on marine biology as a kid, watching “The Undersea World of Jacques Cousteau”! I paid my way through my degrees at San Diego State University by working as a lifeguard. Spending time in, on, under, and around the ocean was just something I wanted (and still want) to do.

Tell us about the mystery of sea urchins and their pits.

MR: Peering into these habitats, I was struck by the close fit between the urchins and the rock cavities or “pits” they occupy. When you see the “hand-in-glove” fit of the urchin to the pit, it is natural to believe that they excavate the pits themselves. And while marine biologists have long assumed that this is the case, there is no hard (pun intended!) evidence conclusively demonstrating that they do. Pure curiosity led me to investigate further.

This tidepool, shown left, with sea urchins in place, and right, after removing the urchins to reveal a “honey-combed” surface, illustrates how snugly the urchins nestle into pits

You wanted to find out if sea urchins really do excavate their own pits. How did you go about this?

MR: It was really a straightforward experiment. I collected rocks, smoothed them, and placed an urchin on each. Then I watched and waited – and hoped that something would happen! After a year, I weighed the rocks and produced 3-D surface plots to quantify pit formation.

What did you find out about sea urchin behavior?

MR: We found that sea urchins do indeed excavate their pits. In fact, the urchins ingest eroded rock particles – essentially, they are eating the rock! We concluded that they erode the pits using their five teeth, which are at the “business end” of a fascinating mouth structure called Aristotle’s Lantern. These teeth chew up algae for food, but also appear to rasp and scrape the surface rock underneath the urchin, eroding the rock. I do not believe the excavation is “intended” to form a pit; I think it is simply a byproduct of their feeding behavior.

A sea urchin held upside down to reveal the five teeth in its Aristotle’s Lantern, surrounded by spines and the tube feet, which grip the rock

While the excavation probably grinds down the teeth surfaces, these teeth are continually regenerating and self-sharpening, so the urchin always has fresh tooth edges to continue the process. The urchin effectively has a regenerating rock pick!

What effect does rock type have on pit formation?

MR: We tested two types of sedimentary rocks: mudstone and sandstone, which mostly differ in the size of the sedimentary particles that make up the stone. We also tested one type of metamorphic rock: granite. While the sedimentary rocks are relatively erodible, the granite is much less easy to erode, and as expected urchins did not excavate detectable granite pits over the year-long experiment. However, the granite rocks did show a measurable weight reduction, indicating that the urchins were causing a low level of bioerosion. In the field, we see urchins in pits on granite, but these pits are shallower than those we see in sedimentary rocks. We estimated that it would take sea urchins more than a century to excavate granite pits!

What most surprised or interested you about your results?

MR: We were not surprised that sea urchins excavate rock – although we were relieved, otherwise our results would have been very boring (pun intended)! What shocked us was how fast they were able to form pits, particularly in the sandstone. It turns out that the rate of bioerosion on sandstone is so great that across their geographic range, sea urchin excavations probably produce as much sediment as an average river delivers to the coast!

MR: In addition to the urchins’ bioerosion shaping the temperate rocky reefs on which they reside, the pits they create provide microhabitats that would not otherwise be available, which likely harbor a distinct set of species. Of course, the pits also have tremendous effects on the urchins – influencing their behavior, growth and “shell” shape as well as their ability to avoid predators and to cling to rocks in heavy surf.

What are the next steps for your research?

MR: There are two avenues of research that could extend our work from the PLOS ONE study. Firstly, I am currently collaborating with engineering and biomechanics experts at Villanova University to study sea urchin teeth: our hypothesis is that the type of rock on which urchins settle dictates the material properties of their teeth as well as tooth regeneration rates. Secondly, I want to expand our study from this Pacific coast species to other temperate sea urchins around the world. Sea urchins occur in pits around just about every continent; we want to examine this bioerosion phenomenon for other species in different oceans. We have a pretty good understanding of the ecological role sea urchins play; I hope that our research will now yield a better picture of their geologic role.

Author

Beth Baker (née Jones)

Beth works at PLOS as Journal Media Manager. She read Natural Sciences, specializing in Pathology, at the University of Cambridge before joining PLOS in 2013. She feels fortunate to be able to read and write about the exciting new research published by PLOS.